Tornadogenesis and the role of a lake breeze boundary on 14 June 2003 in northwest Illinois

Six tornadoes and a funnel cloud were observed over northwest Illinois (Stephenson and Carroll counties) during the afternoon of 14 June 2003. The synoptic-scale environment was similar to that reported with non-supercell tornadoes in Colorado, i.e., moderate instability and weak shear. Little damage was caused by the tornadoes excluding some minor damage to early season crops (F0). However, this was a high visibility event to the general public as it occurred during a Saturday afternoon when many people were outside to witness them.

Showers and weak thunderstorms initiated along a lake enhanced boundary in northwest Illinois. All of the tornadoes occurred in the vicinity of this boundary, and several of them appeared to coincide with the rapid development of the associated convective cell based on WSR-88D reflectivity trends. The storms occurred between 75 and 110 km (40-60 nm) from the radar, so no evidence of their occurrence was apparent in the velocity or spectrum width data.

The tornadoes in this study very closely resemble the Colorado landspouts both with respect to the synoptic-scale background in which they occurred as well as by their visual appearance. In addition, there are many similarities and one key difference between this event and events termed ‘cold air funnels’ by Cooley (1978). Forecast and warning implications of this event will be discussed and compared with Colorado landspout and ‘cold air funnel’ events. Implications of recent work on lake breeze effects on tornadoes in Ontario, Canada will also be addressed.